How do I alter keyframes for a new framerate?
In object mode, select all the objects for which the animation rate needs to be changed from 24 to 60 fps.

In the graph editor, place the 2D cursor at frame zero, and set the pivot center to 2D Cursor

Hit A once or twice to select all of your keyframes. (You may want to hit CtrlH to hide the curve handles if it looks too busy)

Hit S, then X to scale horizontally, then type 2.5, then Enter

This scales your 24 frames-per-second animations out a multiple of 2.5 to 60 frames-per-second.